The MPEG-2 data compression standard, described for instance in “MPEG video coding: a basic tutorial introduction”, by S. R. Ely, BBC Research and Development Report, BBC-RD-1996/3, is not only a standard for digital video broadcasting, but also for storage on digital video disc (DVD). With conventional compact discs (CD), the bit transmission is fixed, and pictures may be CBR (constant bit rate) encoded. Unfortunately, to be encoded at a given quality, two different groups of pictures do not require the same amount of bits. Therefore, when encoding a video sequence, complex pictures will be strongly distorted while simple pictures will have a good quality.
This problem of a varying quality over a same sequence must be avoided with DVD where the best quality is required all along the recording. As a consequence, DVD standard allows for VBR (variable bit rate) transmission of data, which adapts to the picture complexity: as more bits can be spent for complex pictures than for others, the bit rate is varying during a sequence but the quality remains constant.
However, considering the encoding of a video sequence, another difficulty is here added to the constraint of a constant quality: the encoded sequence must fit into a fixed bit budget. In order to respect this additional constraint, it has been proposed, for instance in the international patent application PCT/IB-98/01031 (PHF98560) filed by the applicant on Jul. 6, 1998, to define VBR encoders that perform, prior to the final coding, one or several pre-analysis passes during which a lot of information dealing with the complexity of the pictures is collected and then used in a final pass for an optimization of the bit budget allocation. This means that the video source (e.g. a movie) is fully encoded several times in order to optimize the final recording, which takes a lot of time (for each sequence: number-of-VBR-passes×sequence-duration).